More than 200 Hong Kong Police officers on Wednesday raided the office of pro-democracy online media outlet Stand News and arrested six of its current and former employees for suspected “seditious publications”, Reuters reported.

This is the latest crackdown by the government on independent press in the city following the imposition of a national security law.

Stand News, set up in 2014 as a non-profit, is Hong Kong’s most vocal pro-democracy news outlet since newspaper Apple Daily ceased operations in June following multiple police raids and arrests of several editors.

In a statement on Wednesday, the police said they were authorised to “search and seize relevant journalistic materials”. A video showed officers wheeling suitcases and boxes out of the newsroom.

The police said three men and three women aged between 34 and 73 years have been arrested. However, they did not identify the arrested people.

The South China Morning Post reported that the police had arrested one current and one former editor at Stand News, as well as four former board members, including pop star turned activist Denise Ho and former legislator Margaret Ng.

In a Facebook post, Denise Ho confirmed that she had been arrested and urged friends and supporters not to worry about her.

A video also showed Stand News acting Editor-in-Chief Patrick Lam handcuffed and being taken into the office building.

The Hong Kong Journalist’s Association urged the city’s government to protect press freedom in accordance with the city’s mini-constitution, the Basic Law.

“The police have repeatedly arrested senior members of the media and searched the offices of news organisations containing large quantities of journalistic materials within a year,” it said in a statement.

Benedict Rogers, co-founder and chief executive officer of the non-governmental organisation Hong Kong Watch, said the arrests are “nothing short of an all-out assault on the freedom of the press in Hong Kong.”

In a statement, the organisation said: “When a free press guaranteed by Hong Kong’s Basic Law is labelled ‘seditious’, it is a symbol of the speed at which this once great, open, international city has descended into little more than a police state.”

Pro-democracy activist in exile, Sunny Cheung, also criticised the raids and arrests. “The Stand News is the largest pro-democracy media remaining after the fall of Apple Daily,” he said in a tweet. “Its reports reveal the government’s corrupted behaviors and the agency is thus considered subversive by the authority. Beijing seeks to control the flow of information among the public.”

Hong Kong had been under British control, but was handed back to China in 1997 under a “one country, two systems” principle.